1. Technical Field
This application relates to a laser probe for use in ophthalmic procedures and more particularly to an anamorphic lens in a rotational scanning laser probe for use in ophthalmic diagnosis and therapy.
2. Description of Related Art
State-of-the-art laser ophthalmic therapies scan the retinal tissue with a surgical light or illumination beam. Imaging can be used to guide this scanning of the surgical light beam and to improve its precision. Typically, Optical Coherence Tomography (OCT) is used to image the target tissue, to perform diagnostics, and to combine diagnostics with therapeutic procedures that use a light beam. In an OCT procedure, a laser beam is directed at a spot of a target tissue. The OCT procedure collects data from a range of depths in an axial or A-scan. Data corresponding to different depth can be collected sequentially (time-domain OCT) or simultaneously (frequency domain OCT). Once the A-scan is completed, the beam can be redirected to a new spot where the A-scan is repeated. Scanning along a line of spots transverse to the optical axis of the system, the A-scans can be assembled into a 1+1 dimensional image, called a B-scan. If the spot is scanned along many lines or a line pattern covering an area, such as an X-Y pattern, a 2+1 dimensional image can be assembled, sometimes referred to as a C-scan. In many existing imaging systems the assembly of A-scans into B- or C-scans is simpler if the beam spot is scanned along straight lines.
In some ophthalmic therapies, the OCT imaging systems involve a cannula that is introduced into the eye with its end positioned proximal to the target tissue. The cannula carries inside an optical fiber to deliver an imaging light beam, and also has an optical head in the cannula or distal tip. The light beam is directed by the optical head from the cannula tip to the target tissue. The light beam is directed in a generally forward direction from the tip, forming a soft azimuthal angle (less than 90°) with a longitudinal axis of the cannula (LA). To reduce the invasiveness of the procedure, existing designs employ cannulas with a small outer diameter, for example in the 200-500 microns region. The small diameter of the cannula makes it a challenge to scan the imaging beam over a wide scanning pattern on the ophthalmic target such as over the retina.
Some typical choices of the scanning pattern include linear, raster and circular scanning patterns. The mechanical implementation of circular scanning can be easier and smoother than that of a linear or raster scanning, as a circular scanner can be operated continuously and smoothly to generate a periodic pattern, whereas a linear scanner has to be operated in a back-and-forth manner, resulting in vibrations, large accelerations and corresponding large forces, and thus in a faster rate of wear.
Remarkably, the advantageous linear scanning pattern, preferred because of the simplicity of the image assembly, can be generated by the advantageous circular scanning motion, preferred for its mechanical advantages. One design achieves this by employing two, counter-rotating optical elements, such as two prisms or faceted lenses in a so-called PARS design, as described, for example, in “Ex-vivo Optical Coherence Tomography Imaging of Schlemm Channels with a Scanning Endoscopic Probe,” by Jian Ren, at IOVS, (Feb. 25, 2011).
The drawbacks of this PARS design include that, in order to obtain a linear scan, the two optical elements have to be synchronously rotated and counter-rotated about an axis of the device. The rotation of the two optical elements requires separate support systems for each, and the synchronization of the two rotations requires a complex gear mechanism. Obviously, the complexity of these three systems requires complex controls and also increases the likelihood of malfunction.
Furthermore, the moving elements in the cannula need to be contained in a non-moving exterior sheath to shield the tissue from abrasive contact with the moving parts of the cannula. The support systems, the gear system and the sheath all increase the diameter of the cannula, making the procedure undesirably invasive.